[23:44:03] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 82%
[23:44:16] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 82%
[23:44:28] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 83%
[23:44:42] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 84%
[23:44:58] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 85%
[23:45:14] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 85%
[23:45:14] [Client thread/INFO] [foamfix]: JEI search tree manager injection complete!
[23:45:17] [Client thread/INFO] [FML]: Applying holder lookups
[23:45:17] [Client thread/INFO] [FML]: Holder lookups applied
[23:45:27] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 86%
[23:45:32] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 87%
[23:45:37] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 88%
[23:45:42] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 89%
[23:45:58] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 91%
[23:46:03] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 92%
[23:46:08] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 94%
[23:46:25] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 98%
[23:46:30] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: ItemStack Aspect checking at 99%
[23:46:32] [ThaumicJEI Aspect Cache/INFO] [thaumicjei]: Finished Aspect ItemStack Thread.

Modpack: FTB Sky Adventures
Additional Mods: Optifine, Better FPS (both newest version)
RAM: 5GB
OS: Mac OS Mojave 10.14.4 Beta (18E215a) // Same in Windows 10

** If you need more information, feel free to tell me **

As you can see the aspect checking happens in another process from the client one without holding it back, ThaumicJEI is not the cause as the client will finish loading before the aspect checking happens, it might be your computer specs

In my case, everything (FPS, netcode, opening chests, WAILA...) lags slightly as long as the ItemStack Aspect Checking™ is running. I'm playing on a server, you will probably want to ask for information about my setup, but it won't be helpful to you as it happens in every single setup I've ever had that included ThaumicJEI, and if I provided you with all that information it would constitute Information Overload. Computer specifications are certainly not the problem anyway.

The moment ItemStack Aspect Checking finishes, everything runs smoothly.

The only logical conclusion I've been able to think of is that ItemStack Aspect Checking is computationally expensive, and the rest of the game is forced to wait in line for computing resources. You say the checking runs in a separate thread, and I'm inclined to believe you, but there's always the chance of there being a synchronized block or similar in some function somewhere down the invocation chain that causes performance problems for the main thread anyway.

Could you point me to the precise line where ThaumcraftJEIPlugin.createAspectsFile() is called in a separate thread? I can't seem to find it, and I can't use Eclipse to help me right now.

Edit: Found it.